In the past, it was thought that local area networks for connecting or interconnecting terminals, computers, and peripheral devices such as printers or disc drives over relatively long distances required dedicated shielded cable and amplifiers such that, without such devices, wire runs of only 25-50 feet would reliably permit sufficient signal quality for the transmission of information, whether or not at relatively high data rates. Computers and their peripherals including personal computers manufactured by IBM.RTM., Apple Computer, Inc., e.g. the Macintosh.TM. and Macintosh-Plus.TM. computers and related hard disk drive and laser printer peripherals were and are networked via specialized connector units and cables provided e.g., by Apple Computer, Inc. These connectors require networks, in a given local use area, to be run over separate shielded cables and to "daisy-chain" the devices to be connected together Daisy-chaining refers to the serial connection of devices on the same cable, a party line. Breaking the daisy chain at one link, for example by removing one computer or peripheral and its connector from the line, prevents devices on one side of the break from communicating (through that chain) with devices on the other side of the break and may prevent all devices in that daisy chain from communicating with any other device in the network. For operation of multiple computers sharing the same cables for communication some kind of polling or token passing protocol is required in order that the information can be transmitted, received and properly validated in the presence of other incoming and outgoing signals on the same line. Several network transmission protocols exist allowing multiple computers to communicate over a common signal carrier, a party line.
Interconnect systems for computers and their peripherals (herein cumulatively referred to as "computer products") require the ability to connect together the computer products to allow each to transfer data such as programs, files, mail, etc., from one machine to the next in a convenient manner. The data transfers are made between different types of computers with the aid of a standard file protocol for the network so that, for instance, a Macintosh.RTM. computer and an IBM.RTM. PC can easily share data given the right software for the network. In the past, Apple Computer, Inc. had developed its "Apple Talk".TM. brand network, which allowed several computer products to share expensive or infrequently used peripherals such as modems, printers and mass storage devices. For instance, sharing one laser printer among several microcomputers is one of the most common usages for the Apple Talk connector network. Other uses for the network include electronic mail, sharing or transferring files, and operation of or sharing mass storage devices and other peripherals.
In the past, if one were to transfer data between only two machines, one would simply utilize a compatible cable to plug the serial port of machines together and utilize specially designed serial communication programs such as Mock terminal.TM. or Fee Term.TM. to transfer the files from one machine to the next through the cable. If one wanted to connect more than two computer products, it was common practice to daisy-chain them with more cables and the use of Y adapters. One could even transfer data between any two computers assuming that only one computer was sending and the receiver was ready to receive the data, and assuming that no other transmission was already occurring on the line. State of the art systems, such as the Apple Talk system with its software protocol and its specialized cabling, keeps messages separate and makes sure they are delivered to the intended receiver and that the messages between devices do not interfere. Such systems utilize a transformer to electrically isolate each computer from the signal wires running between the various computers on the network while still allowing each computer to send and receive data over the wires. The rest of the transmission and reception screening protocol is embodied in executable software code.
A major problem of state of the art network systems is the perceived necessity to use shielded cable in order to preserve line quality and the quality of the transmission of the data so that breakup or loss of data would be minimized. Without amplification and other sophisticated cabling techniques specialized for each individual situation, it has been recommended that the use of standard flat multi-wire telephone type cable be limited to runs of 25 feet for use in a ground reference (non-differential) circuit. In laying out the state of the art connector type systems, special modules were utilized and special shielded cabling requiring a special plug was thought to be necessary to assure drop-out-free transmissions. Additionally, jumper boxes were required where, for instance, at a given station there was no applicable computer product. Thus, the Apple Talk cabling system is a serial only system which requires sophisticated hardware and a shielded cable, along with shielded connectors in order, it was thought, to transfer error-free data from one point to another.
It should be noted that, typically, the telephone cable runs throughout an office exceed 1,000 feet and often are as long as 5,000 feet. It was therefore thought that data transmission over standard telephone cables could not accommodate such long runs of cable without modem interconnection.
Other state of the art systems include coaxial cable linked systems of even greater cost and complexity, including, among others, the Xerox Corporation--licensed Ethernet .RTM. system.
It is a principal object of the invention to provide a local area network (LAN) system and modular components therefor affording simple, effective, low cost interconnection among similar or diverse microcomputers and peripherals and over long running lengths of ordinary telephone cable, often sharing a building's existing cables with the telephone system.